1. Field of the Invention
This invention relates to a positive electrode for electrochemical cells and more particularly to a metal oxide electrode which can discharge at a high rate of discharge in primary batteries made of such cells.
Reserve type batteries are one type of primary battery which generally must discharge at high rate; where the energy of the battery is completely removed over a period of only minutes. Divalent metal oxide depolarizers such as silver (II) oxide (AgO) are used in such batteries. The monovalent form of silver oxide, however, is not useable in such applications because of its very high resistivity (10.sup.8 .OMEGA.-cm), even though it has superior stability compared to the divalent oxide. The divalent oxide, however, is more generally used and has a lower resistivity of 10.sup.-60 .OMEGA.-cm but it loses half of its capacity by decomposition to silver monoxide by the reaction: EQU 2AgO.fwdarw.Ag.sub.2 O+1/2O.sub.2
The most generally used silver oxide electrodes are those which are produced by the anodization of porous silver metal in alkali. The electrodes are electronically conductive. A typical example of this class of electrodes is described in U.S. Pat. No. 2,615,930 to Moulton. Plates were made of porous silver by a sintering process. Several other patents including the porous silver electrode patented by Jungner in 1899 (Zinc-Silver Oxide Batteries, John Wiley & Sons, New York, 1971, p.199) and (U.S. Pat. No. 3,002,834 to DiPasquale) describe other methods of producing the sintered silver, but all rely on anodization processes to convert the porous silver to silver oxides.
The sintered electrodes are anodized in alkali for several hours to convert the silver metal powder they contain to the higher valence oxides. It is now well known to those skilled in the art that the anodizing process is only partly efficient and a mixture of silver metal, silver (II) oxide (AgO), and silver (I) oxide (Ag.sub.2 O) is the result of the process. Typically, the amount of AgO thus formed is at best about 85% of the theoretical conversion of the silver metal to the oxide.
A major problem with the process is the fact that the capacity of the electrodes is not consistant in this batch type anodizing method, due to the inevitable variations in current distribution inherent in the process. Additionally, the actual composition of the electrodes cannot be determined directly in a practical, non-destructive manner. Compounding the problems is the previously described fundamental instability of the AgO and the reaction which occurs between the AgO and the unconverted silver metal in the electrode: EQU AgO+Ag.fwdarw.Ag.sub.2 O
Several workers have recognized the problems which are caused by the mixed chemistry of the electrodes. Pure, chemically prepared silver (II) oxide (AgO) has been used by Coleman and King (Power Sources Vol. 1, Pergamon Press, 1967, p.193) who added several percent of carbon in various forms along with other binders, which included methyl cellulose and polyvinyl alcohol. The carbon was added to increase the mechanical strength of the electrodes. The utilization of the active material was about 75-80% at best.
The use of the chemically prepared silver (I) oxide (Ag.sub.2 O) is limited to lower rate applications such as powering watches and hearing aids. The energy of the cells is drained over several days. Even for such low rate applications, conductive additives such as graphite have been added to the oxide to mitigate its high electrical resistance, as described by Passaniti and Megahed (Handbook of Batteries, McGraw-Hill, New York, 2nd Edition, 1995, p.12.1).
In U.S. Pat. No. 4,172,183 to Ruetschi, graphite or silver powder was added to chemically prepared silver (I) oxide (Ag.sub.2 O) or mercuric oxide, functioning as a conductive additive.
In U.S. Pat. No. 4,187,328 to Jumel, silver (II) oxide (AgO) was reduced with hydrazine, which left a coating of silver metal on the surface of the depolarizer particles. The silver coating had several attributes, one of which was the provision of increased electronic conductivity to the electrode. A cell built according to their invention delivered only 74.9% of its theoretical capacity at a low rate of discharge of about 8 ma/cm.sup.2.
U.S. Pat. No. 4,146,685 to Tucholski taught the use of a hydrophillic binder in molded depolarizer tablets, which are pressed into metal containments which act as current collectors. The molded tablets are used in very low rate cells such as those used for watches and hearing aids. The binder had a dual function of a lubricant and a binder, replacing the prior art graphite, which provided the same functions. The electrodes were for use at extremely low rate, but even low rate pulse performance for only 2 seconds at 30 and 100 ohms was inferior to the prior art, using graphite as the additive.
In U.S. Pat. No. 5,580,683 to Takeuchi and Marilla, carbon or graphite and mixtures thereof were used in their preferred embodiment of mixed metal oxide electrodes comprised of silver vanadium oxide, along with a polymeric binder.
All of the referenced prior art which relies on the use of electronically conductive additives or other methods to improve the electronic conductivity of electrodes decrease the energy density of the electrodes, partly because the additives do not take part in the discharge reaction. We believe that they also decrease performance at higher discharge rates because they tend to physically block the active depolarizer from the current collector.
The prior art, which uses types of carbon, which are less electronically conductive than graphite or metal powders, to increase the physical strength of electrodes, similarly decreases energy density because they do not take part in the discharge process and block the active material from the current collector.
2. Summary of the Prior Art
The prior art generally teaches the use of electronically conductive additives to metal oxide electrodes, even for low rate applications. The additives increase the conductivity or the physical strength of such electrodes.
For batteries which must operate at much greater current density, silver (II) oxide (AgO) is used rather than the more stable silver (I) oxide (Ag.sub.2 O) mainly because of its higher conductivity and its initially greater energy content.
The silver (I) oxide (Ag.sub.2 O) is used only with additives which increase the electronic conductivity of the electrodes or increase their strength, and even then they are generally used only for low drain rate applications at current densities of less than a few milliamperes per square centimeter .